Lignocellulosic materials are the most abundant renewable resources that have great potential for production of scalable fuels and chemicals. Extensive attention has been attracted to convert cellulosic biomass to valuable products usually through two step processes: 1) hydrolyze the biomass to sugar monomers; 2) convert sugars into bio-based products (Huber el al. Chem. Rev., 2006, 106, 4044-4098; Zhu et al. Green. Chem., 2006, 8, 325-327). The full potential of the biopolymers has not been fully exploited mainly due to the historical shift towards petroleum-based feedstocks from the 1940s and the recalcitrant nature of biomass, which holds back a cost efficient technology to convert lignocellulosic biomass to sugars (Sun et al. Chem. Commun., 2011, 47, 1405-1421). The use of ionic liquids (ILs) as biomass solvents is considered to be an attractive alternative for the pretreatment of lignocellulosic biomass (Mora-Pale et al. Biotechnol. Bioeng. 2011, 108, 1229-1245). It has been shown that pretreatment with imidazolium based ILs, containing anions such as chloride (Li et al. Ind. Eng. Chem. Res., 2010, 49, 2477-2484), acetate (Li et al. Bioresour. Technol., 2010, 101, 4900-4906) and alkyl phosphate (Brandt et al. Green Chem. 2010, 12(4), 672-679), can greatly accelerate the enzymatic digestion of the pretreated biomass that has been completely or partially solubilized in the IL. Current approaches that use 100% IL as the pretreatment medium require large amount of water to wash out the residue IL in the pretreated biomass and usually the IL is diluted down to below 10%. Thus the conventional IL pretreatment process must also have effective means of recovering and recycling the IL to be cost competitive.
Acid catalysis has been used to produce sugars and other high value compounds in situ through the acid catalyzed hydrolysis of biomass dissolved in imidazolium chloride ILs (Li et al. Green Chem., 2008, 10, 177-182; Rinaldi et al. Angew. Chem., 2008, 47, 8047-8050; Vanoye et al. Green Chem., 2009, 11, 390-396; Sievers et al. Ind. Eng. Chem. Res., 2009, 48, 1277-1286). Li et al reported biomass hydrolysis in ILs with different mineral acids as catalyst and up to 68% total reducing sugars were achieved with the combination of [C4mim]Cl and hydrochloric acid (Li et al. Green Chem, 2008, 10, 177-182). The use of Brønsted acidic ILs to dissolve and hydrolyze cellulose was also reported, where the ILs act as both the solvent and catalyst (Amarasekara et al. Ind. Eng. Chem. Res., 2009, 48, 10152-10155). This could potentially provide a means of liberating fermentable sugars from biomass without the use of costly enzymes. However, the separation of the sugars from the aqueous IL and recovery of IL is challenging and imperative to make this process viable.
Rogers et al reported for the first time that some hydrophilic ILs could form aqueous biphasic system (ABS) in the presence of concentrated kosmotropic salts (Gutowski et al. J. Am. Chem. Soc. 2003, 125, 6632). Since then, significant progress has been made in this field (He et al. J. Chromatogr., A 1082 (2005) 143; Bridges el al. Green Chem 9 (2007) 177; Wu et al. J. Phys. Chem. B 112 (2008) 6426; Neves et al. J. Phys. Chem. B 113 (2009) 5194). It has been reported that IL based ABS can be formed with addition of appropriate amount of K3PO4, K2HPO4, K2CO3, KOH, NaOH, or Na2HPO4 into aqueous [C4mim]Cl (He et al. J. Chromatogr., A 1082 (2005) 143; Bridges et al. Green Chem 9 (2007) 177; Li et al. J. Chromatogr., B 826 (2005) 58). When these kosmotropic ions (anions of the salts) were added into aqueous IL solutions, the hydrogen-bond network of water was enhanced because of their water structuring nature. Therefore, more energy was required for cavity formation around the bulky organic [C4mim]+ cation. At a certain concentration of kosmotropic salts, an aqueous phase containing IL with more hydrophobic cation and less water-structuring anion was separated (Li el al. J. Wang Trend Anal Chem, 2010, 29, 1336-1346).
The present invention provides a process to utilize IL phase separation behavior to efficiently extract sugars from aqueous ILs. Surprisingly, sugar and IL recovery can be realized in a single step.